The Short Answer
Commercial kitchen exhaust is designed around the cooking equipment it serves: engineers select the correct hood type (Type I for grease-laden vapors, Type II for heat and moisture), calculate the required exhaust airflow, size and route grease duct with proper clearances and slope, and provide balanced make-up air. The design follows the International Mechanical Code, including IMC Chapter 507 for commercial kitchen hoods, and is documented in stamped mechanical drawings for permit.
Type I vs. Type II Hoods
The first design decision is hood type. Type I hoods are required over equipment that produces grease-laden vapors and smoke — fryers, griddles, ranges, charbroilers. They include grease filters and connect to grease duct. Type II hoods serve equipment producing heat, steam, or moisture without grease — dishwashers, ovens, and steam kettles. Selecting the right hood type for each appliance is fundamental to a code-compliant, safe kitchen ventilation system.
Exhaust Airflow and Hood Sizing
Exhaust airflow is calculated based on the hood style, the type and duty of the cooking appliances beneath it, and the hood dimensions and overhang. Higher-duty equipment such as charbroilers requires more exhaust than light-duty ovens. The hood must physically extend beyond the cooking surface to capture effluent. Engineers document the required airflow for each hood and select fans that deliver it while accounting for duct losses.
Grease Duct Design
Grease duct carries hot, grease-laden exhaust from Type I hoods to the exhaust fan, usually on the roof. Design requirements include liquid-tight welded construction, minimum clearances to combustibles or approved enclosure, slope for drainage back to the hood or a grease reservoir, and access for cleaning. The duct routing is coordinated with structure and other systems, and the fan is selected to overcome the total system resistance. These requirements come from the mechanical code and fire-protection standards.
Make-Up Air
Air that leaves through the hood must be replaced. A large exhaust system without adequate make-up air depressurizes the building, causing doors that won't open, backdrafting of gas appliances, and poor hood capture. Engineers provide make-up air — often through a dedicated make-up air unit (MAU) that can temper the incoming air — sized to balance the exhaust. Balancing exhaust and supply is a core part of kitchen ventilation design.
When Do You Need This?
Any commercial kitchen with cooking equipment that produces grease or significant heat and moisture needs engineered exhaust design for permit. This includes restaurants, cafeterias, ghost kitchens, break rooms with commercial cooking, and food-service tenant improvements. Even replacing or reconfiguring existing cooking equipment can trigger a re-evaluation of the hood, duct, and make-up air.
Frequently Asked Questions
Can I use an existing hood for new equipment?
Sometimes, if the existing hood is the correct type and size for the new appliances and the exhaust and make-up air remain balanced. New or higher-duty equipment often requires re-engineering the system, which we can evaluate.
What is IMC 507?
IMC Chapter 507 is the section of the International Mechanical Code governing commercial kitchen hoods — hood types, capture, exhaust, and related requirements. It's a primary reference for kitchen ventilation design, along with fire-protection standards for grease duct.
Does the exhaust design coordinate with fire suppression?
Yes. Kitchen hoods require a fire-suppression system, and the mechanical design coordinates with the suppression contractor and structural support for rooftop equipment. We coordinate these interfaces as part of the MEP package.